Magnetohydrodynamic power system with semi-closed cycle

ABSTRACT

An MHD power system is provided in which the working fluid consists of combustion gases that are recycled through an MHD generator with new fuel and oxidant being added and an equal mass of gas being discharged at some point in the cycle to provide what may be called a &#39;&#39;&#39;&#39;semi-closed cycle.&#39;&#39;&#39;&#39; Such a cycle provides improved electrical conductivity of the working fluid as compared with open cycle systems that are operated with excess oxidant while it also provides increased mass flow as compared with open cycle systems that are operated without excess oxidant.

FIF85U2 United States Patent Way [ lMarch 13, 1973 Primary Examinerl).X. Sliney Attorney-A. T. Stratton, F P. Lyle and Gordon H.

ABSTRACT An MHD power system is provided in which the working fluidconsists of combustion gases that are recycled through an MHD generatorwith new fuel and oxidant being added and an equal mass of gas beingdischarged at some point in the cycle to provide what may be called asemi-closed cycle." Such a cycle provides improved electricalconductivity of the working fluid as compared with open cycle systemsthat are operated with excess oxidant while it also provides increasedmass flow as compared with open cycle systems that are operated withoutexcess oxidant.

11 Claims, 6 Drawing Figures HEATER] {I5 4 [75] Inventor: Stewart Way,Pittsburgh, Pa. Tower [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. 57

[22] Filed: April 30, 1970 [21] Appl.N0.: 33,430

[52] U.S.Cl ..3l0/ll [51] Int. Cl. ..H02n 4/02 [58] Field of Search..3l0/l l- [56] References Cited UNITED STATES PATENTS 3,467,842 9/1969Caprasse ..3l0/ll 3,223,860 12/1965 Brill ..3l0/ll OXIDANT P l4 FUEL ,1T COMBUSTION l8 CHAMBER l RECYCLED PRODUCTS MOTOR PATENTEUMAR 1 31975720, 50

SHEET 10F 2 OXIDANT PMHD HEATER l5 J2 l4 l6 1 FUEL MHD I] lo MCOMBUSTION CHAMBER 1 RECYC/L'ED PRODUCTS MOTOR SEED FUEL MAKE-UP{RECYCLED PRODUCTS BURNER PMHD (I9 25 |2 T T 16 I51 HEA TER I H K M MCLEANER MIXER} MHD GEN. IQ

' j J STEAM l7 PLANT AIR MR STACK CooLER M 24 20 RECYCLED I I SEED A 255CHAR. OXYGEN P A BOI ER 20 Co RICH MHD l5 HE TER L '9 J I GAS W s s r SCOMB. M/W] /v] ,/25 REACTOR IO CLEANER A A SEED P5 T IT I w w I ASH 222| PATENTEDMARIBIQYS 3,720,850

SHEET 2 OF 2 2o 0 ls EATEr (BOILER fig 8 1 PMHD MN (l2 lcHAR.

/C CLEANER \A/vv wvw A -25 I MH REACTOR I 9 SEED hAsH 22 2| LFUEL GASPRODUCTS H? s -FUEL GAS paooucrs FIG.4. 24 m CHAR.

MHD I5 I 30 I2 1 (HEATER (BOILER l9 coma. MHD

4 m M 125 REACTOR I [AIR CLEANER PR5ODUCTS M PS I l P m ASH 22 i AIR 1Fl 6.5.

CHAR.

EATER BOILER AND CLEANER REACTOR MAGNETOHYDRODYNAMIC POWER SYSTEM I WITHSEMI-CLOSED CYCLE BACKGROUND OF THE INVENTION 1. Field of the inventionThis invention relates to magnetohydrodynamic power systems in which anEMF is derived by the motion of a conductive working fluid within amagnetic field. The term magnetohydrodynamic is sometimes abbreviated asMHD.

2. Description of the Prior Art MHD systems are known that use an opencycle. There are also known types of MHD systems that utilize a closedcycle. The open cycle systems are characterized by fuel and oxidantsupplied to a combustion chamberwith the products thereof supplied tothe MHD generator and subsequently exhausted. In closed systems, theworking fluid is recirculated through the MHD generator in a closedloop. The working fluid in such closed systems may be heated either by anuclear reactor or by an externally fired heater. The working fluiditself is usually chosenfrom among the inertgases such as helium andargon. Both open and closed systems each have various advantages anddisadvantages.

In general, open cycle systems have drawn the most interest for earlypractical application. As technology advances, it is possible to'preheatthe oxidant gases to increasingly high temperatures. This leads to animprovement in the overall efficiency of the system. The ability topreheat to a higher temperature suggests that the fuel to oxidant ratiocan be made smaller so that the result is greater efficiency in terms ofpower generation per unit of fuel supply. It is known that the generatedpower is proportional to the mass flow of gas through the system. Thus,it would appear that, employing established concepts of the MHD art, itwould be advantageous to use large quantities of oxidant, such as 75 or100 percent excess air compared with that necessary for stoichiometriccombustion of the fuel, when a preheat temperature such as within therange of about 2000to 3000 K is attainable.

Another modification that is made possible as higher preheattemperatures are attainable, still following existing concepts, is tokeep the fuel to air ratio essen tially unaltered but simply allow thecombustion temperature' to assume the higher equilibrium value which ismade possible by the higher preheat. This leads to more power beingextractable from the gases expanding through the MHD generator, andhence more power produced per unit of fuel burned. Though this method ofreaping a gain from higher preheat is possible it has technicaldisadvantages because of structural and durability problems.

Another relevant aspect of current arts pertains to gas turbines. in gasturbine technology a number of different types of gas cycles areemployed including open, closed and semi-closed cycles. Significantdifferences in criteria exist, however, for the selection of aparticular type of cycle in a gas turbine system as compared with theselection of a particular type of cycle in an MHD system. For example,in a gas turbine cycle it may be desirable to build up the pressurelevel of the operating gas throughout the system, and this may befacilitated by the use of a semi-closed cycle. This leads to morecompact component sizes, while still retaining the simplicity ofinternal combustion for heat addition. Such purposes do not apply to thefuel burning MHD system and therefore from the state of the art existingprior to the present invention there was no obvious reason or advantagefor employing a semi-closed cycle in an MHD power system.

SUMMARY OF THE INVENTION According to the present invention, asemi-closed cycle is used in an MHD power system. By a semiclosed cycleis meant one in which new fuel and oxidant reactants are supplied to thesystem at the same time material is recirculated in the system, with adischarge of gas corresponding to the rate of additional new materialadded. The invention resides in part in the fact that while it isadvantageous to increase the mass flow in the system for greatest powergeneration, if one seeks to reap benefits of higher preheat temperature,to do so through the expedient of supplying excess oxidant forcorresponding increase in overall efficiency results in a seriousdrawback, namely, that as the oxidant to fuel ratio is increased, theelectrical conductivity of the gas markedly decreases, at specifiedpressure and temperature. a

The present invention circumvents the difficulty just mentioned. Byrecycling the combustion products, the conductivity is thatcorresponding to a near stoichiometric combustion products mixture. Theconductivity of the mixture is very important as has been previouslyrecognized in MHD power studies.

The advantage of recycling is increased when the fuel and oxidant are inclose to stoichiometric proportions, or even on the fuel-rich side, andwhere such fuel and oxidant relation exists a power plant of minimumsize can be provided for given power generation. Thus, use of recyclednear-stoichiometric products yields a much more compact generator thanuse of a large amount of excess air. On the other hand, the benefit ofproduct gas recycling can be taken in the form of higher pressure ratiosand higher cycle efficiency ifv one prefers not to reduce the generatorlength. Still another option, if gas recycling is used, is a choice ofwhether to secure the advantage of a lower preheat temperature or ashorter MHD generator duct while keeping efficiency constant.

If raw oxygen were used as oxidant the attainable flame temperature isvery high. By recycling combustion products as taught in this inventionthe operat ing temperature is somewhat reduced, and the mass flow isincreased. Efficiency is improved by virtue of the larger mass flow, andthe electrical'conductivity is superior to what it would be if simplyextra diluent air were blended with the oxygen.

Another advantage of using recycled products rather than excess air isthat the formation of nitric oxide is very considerably reduced. Thisnitric oxide (NO) is a disagreeable air pollutant.

A further advantage of recycling the combustion products, irrespectiveof the use of high air preheat temperature, pertains to the generaldesirability of running with excess fuel to obtain better electricalconductivity. Without use of products recycling, the mass flow per unitof fuel burned is reduced when we burn with excess fuel, and thisdegrades plant efficiency. By recycling some combustion products thisdefect in mass flow can be removed.

The desirability of recycling combustion products in combustion firedMHD plants may have been missed, in the past, because MHD engineers (I)generally associated higher air preheat temperature with higher flametemperature; (2) they were not fully aware of the marked effect ofexcess air on lowering the conductivity; or (3 they would tend to regardthe semi-closed cycle in the framework of gas turbine technology wherehigh pressure is advantageous, whereas very high pressures aredisadvantageous in MHD cycles.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of asimple MHD power system embodying the present invention; and

FIGS. 2 through 6 are schematic diagrams of more specific embodiments ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an MHDgenerator is shown from which electrical power, PMI-ID, is derived, byreason of flow through it, in the presence of a magnetic field, of aworking fluid that is conductive. The invention is most advantageouslyemployed in a system in which the working fluid consists of gaseousproducts of the combustion of a fuel. Such fuels include, as examples,coal, oil, natural gas, gas produced from coal, carbon monoxide, charand various hydrocarbon fuels. Such fuels arerelatively inexpensive andplentiful and provide good possibilities for MHD power systems that areeconomical.

In the system of FIG. 1, such a fuel and an oxidant, such as air,oxygen, or oxygen enriched air, are supplied to a combustion chamber 12in which the fuel is burned and gaseous products thereof aresubsequently applied to the inlet 14 of the MHD generator. (If air is aconstituent of the oxidant, air preheating is generally required, but itis not-shown in FIG. 1.) The gases emitted from the outlet 16 of the MHDgenerator, essentially unchanged in form from those supplied at theinlet, are in accordance with this invention recycled through loop 18from the outlet 16 back to the inlet 14 of the MHD generator where theyare mixed with new combustion products and reintroduced into thegenerator. An amount of gas is discharged from the system, such as tothe atmosphere through a stack 20, equal in mass, at least oversubstantial periods of time in which the system reaches a state ofequilibrium, to the quanti ty of combustion products newly introduced asa result of combustion in the combustion chamber 12. The recycledcombustion products are, in recycling path 18, compressed by compressor17 and preheated in heater 15.

Systems in accordance with this invention, as exemplified by thatschematically shown in FIG. 1, may be properly described as systemsoperating on a semiclosed cycle." A quantity of the utilized material isrecirculated and reused but is however mixed with new material. Theratio of recycled to new combustion products employed in the system maybe widely varied, depending on the amount of preheat employed and valuesof other selected parameters. Although it is conceivable'to operate anMHD system on a closed cycle which might employ fuel combustion productsthat after an initial period of production in the system are then sealedoff, such a system would result in disadvantages of a very hightemperature of operation of the heat exchanger where the working fluidis heated and added thermodynamic losses in the heat of the gases 7passing from the furnace to the stack. Although MHD power systems havebeen previously disclosed that operate on open cycles and others thatoperate on closed cycles, it has not been previously disclosed tooperate an MHD power system on a semi-closed cycle. Furthermore, whilethe concept of a semi-closed cycle is well known in gas turbinetechnology, it is used for reasons inapplicable to MHD power systems.The present invention results in part from the fact that it is newlyrecognized that there are distinct advantages for operating an MHD powersystem on a semi-closed cycle.

It is known that the power derived from an MHD power system is directlyproportional to the specific enthalpy drop of the working fluid as itpasses through the energy converter and also to the mass flow of workingfluid through the system. If is of course desired to maximize the powergenerated per unit of supply fuel. The relation of MHD power and massflow suggests that an improvement in generated power, and the overallefficiency of the power generation process since no additional magneticfield is required, may be achieved by the supply of excess oxidant inthe working fluid in order to increase the mass flow in the system.Fitting with this is the fact that with advancing technical progress itis possible to heat gases to increasingly high temperatures.Regenerative heat exchangers have been proposed for this purpose. Thismakes it possible to increase the temperature of the oxidant with theresult that the fuel to oxidant ratio can be reduced while stillachieving high efficiencies, perhaps over 60 percent. Theseconsiderations would suggest that as the permissible oxidant preheattemperature is raised, more excess oxidant may be used for a goodoverall power generation efficiency. More careful consideration,however, reveals that there is a major difficulty that is over-looked inthe foregoing considerations and that is that as the oxidant to fuelratio is increased, the conductivity of the gas decreases.

The difference in conductivity can be shown by considering a systemwherein air is supplied with the fuel in an amount of percent in excessof that required for stoichiometric combustion with the fuel. With agenerator inlet static pressure of 3.5 atmospheres and a generator inlettemperature of 2600 K, and 0.7 percent by weight cesium seeding, it canbe shown by calculations that the generator inlet conductivity is about6 mhos per meter. (Calculating the conductivity of a gas withconsiderably accuracy is possible from fundamental physical concepts.Reference may be made for an example of such calculations to a paper byL. S. Frost appearing in the Journal of Applied Physics, V. 32, (1961),p. 2029 and also to the paper of LS. Tuba, R.L. Chambers, W.E. Young andS. Way in April, 1965, Transactions of the ASME, Journal of Engineeringfor Power, V. 87(A), p. 125.)

On the other hand, it can be similarly shown that with the same systemunder all of the same conditions, except that now the air and fuel existin a stoichiometric mixture, the generator inlet conductivity is about10 mhos per meter. That is, this conductivity would result in aconventional open cycle system in which fuel and oxidant arecontinuously supplied and discharged subsequent to passing through theMHD generator.

In accordance with this invention the system utilizes recycledcombustion products for increased mass flow without sacrifice inconductivity in that new fuel and oxidant may be supplied instoichiometric proportions. Such a mixture of recycled combustionproducts and new products provides the same inlet conductivity, in theabove example, of about mhos per meter. In the second case, whererecycled combustion products take the place of the 75 percent excess airof the first case, the mass involved is nearly the same. Yet theconductivity at generator inlet is about 70 percent higher in the secondcase than in the first. Temperature levels are essentially the same inboth cases, both for preheat and combustion chamber temperatures. Plantefficiencies will be over 60 percent in both cases. However, while theMHD generator length (with appropriate design assumptions), of theexample with excess air is about 17 meters, that of the example withrecycled combustion products will be only about 10 meters. Such ashorter length greatly reduces the cost of the superconducting magnetwhich is one of the major expense items of the power plant.

The advantage of the higher conductivity resulting from the use ofstoichiometric combustion products can be taken in another way. This is,without reducing the generator length, the pressure ratio of inlet tooutlet of the generator duct may be increased and thereby the plantefficiency is raised. Another alternative advantage that could be takenof the recycling feature is to permit the use of lower preheattemperatures. It can be seen that the present invention gives the systemdesigner considerable flexibility in choice of factors of system economyand efficiency.

The improved electrical conductivity in systems in accordance with thepresent invention as compared with those using gas mixtures with a largeamount of excess oxidant can be accounted for from the excessiveformation of negative OH ions in the latter case. When each negative ionis formed there is effectively a removal from the system of a conductionelectron. The following table results from calculations of equilibriumcomposition, electron mobility, electron density, and electronconductivity for systems in which char is used as the fuel, burned withmoist air, with 0.7 percent by weight cesium seeding and the gas stateat 1 atmosphere and 2300 K, typical of the generator outlet conditions.

stoichiometric Air and Fuel 75% Excess Air conductivity (mhos/m.) 5.l422.285 mobility (m'lv. sec.) L233 1.324 electron density (m.'3) 2.61 XI0" [.08 X lo mole fraction OH 0.00190 0.00291 mole fraction 0 0.000470.00142 mole fraction Cs 0.000553 0.000402 mole fraction NO 0.003320.01019 mole fraction NO, I X 10- 7 X 10- The higher conductivity forthe instance in which no excess air is supplied must be accounted for bythe higher electron density. This higher electron density can onlypartially be accounted for by the higher concentration of cesium atoms(conductivity is proportional to the square root of cesiumconcentration). The major contributing factor is concluded to be thelower concentration of ionic species of OH, O, NO and N0 Since OH is themost prominent negative ion species, and since NO is of littleconsequence because of its low electron affinity (0.5 e.v. compared to1.83 e.v. for OH) it is believed that OH is chiefly responsible,

through formation of OH, for the lower conductivity of the gas in theright hand column.

It is to be understood that, in systems incorporating the presentinvention, it is not essential that the combustion reaction occur with astoichiometric mixture of fuel and oxidant. It is known, for example,that an excess of fuel would be beneficial to electrical conductivity.An excess of oxidant is tolerable but generally speaking greateradvantage is taken of the recycling of combustion products when themixture is near to stoichiometric or slightly fuel rich. Thus, torealize the advantages of the present invention it is not necessary toset the ratio of fuel and oxidant to any narrow limit or at any specificvalue. Generally speaking, substantial advantage can be taken of thepresent invention in systems operating with fuel and oxidant supplied inquantities within a least 10 percent of stoichiometric quantities oneither side, fuel rich or oxidant rich.

The expedient of seeding the working fluid with an alkali metal such ascesium or potassium is contemplated for use in systems in accordancewith this invention as has been in prior use in MHD systems. That is,the considerations concerning the need for such a seeding material inorder to increase the conductivity of the working fluid remain the samehere as before. The ex tent of seeding, the extent of preheating ofoxidant and recycled combustion products and the mixture ratio of theworking gas all have to be with the achievement of a particular desiredconductivity of the working fluid which in a general case should be atleast 5 mhos per meter in systems of practical interest. It is knownthat a regenerative heater with a matrix of high purity magnesium oxidecan be built to function effectively with ash free gases at preheattemperatures of about l950 K. Description of such a heater may be foundin an article by F. Hals and L. Keefe entitled A High TemperatureRegenerative Air Preheater For MHD Power Plants presented at theInternational Symposium on MHD Electrical Power Generation, Salzburg,Germany, July l966. If such high temperature air preheating is used thepresent invention becomes particularly attractive, although it can alsobe used advantageously with more moderate air preheating.

Referring now to FIG. 2, there is shown in more detail an example of asystem in accordance with the present invention. Here there is shown aburner or combustion chamber 112 in which fuel is burned with air andthe combustion products introduced into a mixing chamber 13 to which isalso supplied alkali metal seeding material and recycled seed andcombustion products. The mixture is then supplied to the inlet 14 of anMHD generator 110. The working fluid emitted from the outlet 116 of theMHD generator goes to a heater l5 and provides a medium for heating airto be supplied to the burner through path l7 and also for heatingcombustion products that are recycled through path 18 just prior tobeing introduced into the mixing chamber 13. Thereafter, there may beprovided a steam plant 19 which is optional but is generally preferredto derive a certain additional amount of power P (A gas turbine plantcould be used in lieu of the steam plant.) Power from the steam plant 19can be used to drive compressors 21 and 22 for the recycled combustionproducts and for air taken in from the atmosphere, respectively.Subsequent to the steam plant 19 the material is passed through acleaner 25 in which the alkali seed compounds and any ash or othercontaminants are removed and the seed material is recycled through path23 while the combustion products are recycled after first passingthrough a cooler 24. A stack is provided for the discharge from thesystem of a mass of gas equivalent to that introduced by the fuel andair supply to the combustion chamber.

In the described system of FIG. 2 there are illustrated separate burnerand mixing chambers 12 and 13 which may be desirable in order to insureagainst having the combustion occur in a vitiated oxygen atmosphere,although it is contemplated that the combustion as well as the mixing ofthe various gases may all take place within a single chamber since thetemperature is high enough to cause chemical reactions to proceed athigh velocity.

The system of FIG. 2 contemplates the use of air as the oxidant which isa matter of choice for the designer. If pure oxygen is used as theoxidant, rather than compressed air, a further improvement inconductivity is realized because the moisture ordinarily present inatmospheric air is eliminated. However, there is encountered thedisadvantage of having an oxygen plant to separate oxygen from air butthat disadvantage would be offset to some degree by a great reduction inthe required power of the compressors. Oxygen enriched air, that is,amixture of air and oxygen, may also be used as the oxidant.

New fuel and oxidant are suitably supplied sufficient to producecombustion gases in a ratio of from about 0.05 to 5.0 parts of recycledcombustion products to 1 part new combustion products, where the oxidantis raw oxygen. Where the oxidant is air, about 0.05 to 2 parts recycledcombustion products to 1 part new eombustion products is suitable.

As previously mentioned herein, any of a variety of fuels may be used insystems in accordance with this invention. Among those presently ofinterest are low cost fuels derived from coal. Char is a fuel derivedfrom coal by removal of a large portion of volatile constituents. It maybe available in the future in fairly large quantities in the course ofproduction of gaseous or liquid fuels from coal. It is especiallyattractive as an MHD power plant fuel because of its low hydrogencontent which leads to superior electrical conductivity in the productgases when alkali seed material is added. Conceptual studies have beenmade of char burning MHD systems in which combustion products arerecycled. These studies are reported in a paper by the present applicantentitled Char Burning MHD Systems and available as a preprint from theAmerican Society of Mechanical Engineers, paper 69-WA/Ener- 13., WinterAnnual Meeting, November 1969. Reference should be made to that paperfor analysis and discussion of such systems.

FIGS. 3 through 6 illustrate char burning MHD systems that will bebriefly described by way of further example of the present invention. Itshould be understood of course that in considerable part the descriptionapplies to systems utilizing fuels other than char. Systems using coalitself may be designed in a manner similar to those of the char system.

In char burning systems, the char may be reacted with hot carbon dioxideto give a fuel gas rich in carbon monoxide. The carbon monoxide thenacts as the fuel in the main combustion chamber. This procedure has theadvantage of providing a clean gaseous fuel and permitting a very highpreheat temperature since preheaters can operate hotter with cleangases. An alternate procedure is to burn the char or coal in a specialcombustion chamber which rejects most, but not all, of the ash. Thepresent invention is still applicable although preheat temperature andattainable efficiency will not be so high. U.S. Pat. No. 3,358,624 bythe present applicant may be referred to for description of a suitablecombustion chamber.

In FIG. 3 there is shown a system in which a char reactor 30 is ahead ofthe MHD generator 10. From the char reactor 30 a CO rich gas is suppliedto a combustion chamber 12 for combustion, with oxygen, producingproducts that are approximately percent CO and 8% H O. This gas, afterdoing its work in the generator 10, goes through the heat recoverysystem including a heater 15 and a boiler of a steam plant 19. Then aportion goes to the cleaner 25 and to the stack 20 while the otherportion after scrubbing is recycled back to the reactor combustorsystem. To achieve the best heat rate, it is desired to have maximumpreheating and as large a ratio as possible of recycled gas to oxygen.Such a ratio can be as high as 4 to 1.

In FIG. 4, the char reactor 30 is placed downstream of the MHD generator10. The stream leaving the MHD generator 10 consists of three partsdesignated A, B and C. A first part A goes to the char reactor 30. Asecond part B is discharged through the stack 20 (preferably removingparticulate matter) and a third part C is recycled as diluent in thesystem. The latter two portions are diverted together immediately afterthe MHD generator 10. Part C is recombined with the portion A that goesthrough the reactor 30 after going through heat recovery elements 15 and19. For best heat rate, maximum exhaust heat recovery must be obtainedand the compressor 22 should operate at the lowest possible inlettemperature. It has been proposed to use a molten salt bath in the charreactor for mechanical and heat transfer reasons. Such a molten saltbath may also serve as a seed trap, making further seed recovery fromthe stream unnecessary although as shown a scrubber may be used in thesystem. The arrangement of FIG. 4 employs what is sometimes referred toas chemical regeneration. That is, a portion of the heat of the exhauststream comprises the heat of endothermic reaction of the fuel and carbondioxide to form a fuel gas. FIG. 4 shows such a chemical regenerationsystem combined with the gas recycling as taught in the presentinvention.

In FIG. 5 an air operated system is shown that is similar to that ofFIG. 4 with however the char reactor shown in front of the combustionchamber in a manner similar to that of the oxygen operated system ofFIG. 3. In FIG. 6 another air operated system is illustrated that isintended to operate on the slightly fuel rich side, about 0.95 percentof stoichiometric air to fuel ratio. Additionally the system of FIG. 6includes the char reactor and combustor together which would be adesirable expedient although they may be physically separated.

These various systems have all been subjected to detailed analysis whichis reported in the aforementioned publication by the present applicantand all include as a part thereof the recycling of combustion productsin accordance with the present invention.

Although the systems portrayed in FIGS. 3 through 6 indicategasification of the char or coal, it should be kept in mind that, atleast in FIGS. and 6, a combustion system could be used which does notnecessarily first gasify the fuel. Thus, coal or char can be burned in amulti-stage cyclone combustor rejecting 90 to 95 percent of the ash.

In the foregoing discussion it will be understood that when we speak ofventing to the stack an amount of gas equal in mass to the mass of newfuel and oxidant continuously added, we also imply venting the smallamount of seed material that inevitably escapes through the cleaner, andwhich is balanced by the makeup seed material injected ahead of the MHDgenerator.

The drawing and descriptions of the arrangement of components in the MHDpower plants given here do not reflect details of minor auxiliaries suchas pumps, coolers, feedwater heaters, wall cooling coils and the like,and it will be understood that such drawings are schematic, and thatabsence of such details does not limit the type of power plant to whichthis invention of semi-closed cycle MHD operation is applicable.Moreover the indication of the sequential arrangement of heatexchangers, particularly as between boiler and oxidant and recycledproduct heaters, is not an essential feature of the invention and issubject to adjustment, modification and rearrangement by a designer whois versed in the MI-ID art.

Iclaim:

1. In a MI-ID power system: an MIID generator; means to burn fuel withoxidant and supply combustion products thereof to said generator; meansto recycle combustion products that have passed through said MI'IDgenerator for reintroduction therein, while venting a flow equivalent tothe rate of supply of new fuel and oxidant.

2. The subject matter of claim 1 further comprising: means to heat saidoxidant prior to combustion with said fuel and also to heat saidrecycled combustion products before reintroduction in said MHDgenerator.

3. The subject matter of claim 1 wherein: said means to burn fuel withoxidant and supply combustion products thereof to said generatorcomprises a combustion chamber with means to supply fuel and oxidantthereto for combustion therein and a mixing chamber with means to supplycombustion products from said combustion chamber to said mixing chamber,means to supply said recycled combustion products to said mixingchamber, means to supply an alkali metal seeding material to said mixingchamber, and means to supply mixed gaseous material from said mixingchamber to said MHD generator.

4. A semi-closed cycle MI-ID power system comprising: an MHD generatorhaving an inlet and an outlet for a working fluid having conductivityand means to derive an EMEresulting fro m flow of said working fluidbetween said inlet an said outlet; a chamber connected with said inletto form therein said working fluid and to supply said working fluid tosaid inlet, said working fluid comprising (1) a quantity of fuelcombustion products that have not previously been cycled through saidsystem, (2) a quantity of fuel combustion products that have previouslybeen cycled through said system, and (3) an alkali seeding material; aheater chamber connected with said generator outlet with means to heatoxidant prior to fuel combustion and to heat combustion products priorto recycling; means to permit discharging from said system a quantity ofgas equal in mass to said quantity of fuel and oxidant that have notpreviously been cycled through said system.

5. A method of operating an MI-ID power system comprising the steps of:supplying combustion gases resulting from fuel combustion to an MI-IDgenerator, said gases including new combustion gases resulting fromcombustion of newly introduced fuel and combustion gases that arerecycled combustion products; and discharging a quantity of gas from thesystem that is equal in mass to the new combustion gases supplied.

6. The subject matter of claim 5 wherein: said combustion gases areformed from fuel combustion with air that has been preheated prior tosaid combustion and said recycled combustion gases are also heated priorto reintroduction in said generator.

7. The subject matter of claim 6 wherein: said combustion gases aresupplied in a ratio of from about 0.05 to 5.0 parts recycled combustionproducts to 1 part new combustion products.

8. The subject matter of claim 5 wherein said new combustion productsare generated by first producing an ash free gaseous fuel throughreaction of a coal derived fuel with all or a portion of said combustiongases that are recycled, and burning said ash free gaseous fuel withoxidant in a combustion chamber ahead of the MI-ID generator.

9. The subject matter of claim 5 wherein said new combustion productsare formed by reaction of a coal derived fuel with preheated oxidant ina combustion chamber designed to separate out a major fraction of theash.

10. The subject matter of claim 5 wherein the recycled combustionproducts are introduced into the combustion chamber in which reaction offuel and oxidant is taking place, so that mixing of said recycled gasesand chemical reaction of fuel and oxidant take place simultaneously andin the same chamber.

11. The subject matter of claim 5 wherein the fuel is first caused toreact with the combustion products leaving the MI-ID generator to form agaseous fuel, and wherein said gaseous fuel is then burned in acombustion chamber with oxidant ahead of the MHD generator, and whereinadditional recycled products which are compressed and preheated are thenintroduced into and mixed with the combustion products coming from saidcombustion chamber prior to entering the MI-ID generator.

# 1* III k

1. In a MHD power system: an MHD generator; means to burn fuel withoxidant and supply combustion products thereof to said generator; meansto recycle combustion products that have passed through said MHDgenerator for reintroduction therein, while venting a flow equivalent tothe rate of supply of new fuel and oxidant.
 1. In a MHD power system: anMHD generator; means to burn fuel with oxidant and supply combustionproducts thereof to said generator; means to recycle combustion productsthat have passed through said MHD generator for reintroduction therein,while venting a flow equivalent to the rate of supply of new fuel andoxidant.
 2. The subject matter of claim 1 further comprising: means toheat said oxidant prior to combustion with said fuel and also to heatsaid recycled combustion products before reintroduction in said MHDgenerator.
 3. The subject matter of claim 1 wherein: said means to burnfuel with oxidant and supply combustion products thereof to saidgenerator comprises a combustion chamber with means to supply fuel andoxidant thereto for combustion therein and a mixing chamber with meansto supply combustion products from said combustion chamber to saidmixing chamber, means to supply said recycled combustion products tosaid mixing chamber, means to supply an alkali metal seeding material tosaid mixing chamber, and means to supply mixed gaseous material fromsaid mixing chamber to said MHD generator.
 4. A semi-closed cycle MHDpower system comprising: an MHD generator having an inlet and an outletfor a working fluid having conductivity and means to derive an EMEresulting from flow of said working fluid between said inlet and saidoutlet; a chamber connected with said inlet to form therein said workingfluid and to supply said working fluid to said inlet, said working fluidcomprising (1) a quantity of fuel combustion products that have notpreviously been cycled through said system, (2) a quantity of fuelcombustion products that have previously been cycled through saidsystem, and (3) an alkali seeding material; a heater chamber connectedwith said generator outlet with means to heat oxidant prior to fuelcombustion and to heat combustion products prior to recycling; means topermit discharging from said system a quantity of gas equal in mass tosaid quantity of fuel and oxidant that have not previously been cycledthrough said system.
 5. A method of operating an MHD power systemcomprising the steps of: supplying combustion gases resulting from fuelcombustion to an MHD generator, said gases including new combustiongases resulting from combustion of newly introduced fuel and combustiongases that are recycled combustion products; and discharging a quantityof gas from the system that is equal in mass to the new combustion gasessupplied.
 6. The subject matter of claim 5 wherein: said combustiongases are formed from fuel combustion with air that has been preheatedprior to said combustion and said recycled combustion gases are alsoheated prior to reintroduction in said generator.
 7. The subject matterof claim 6 wherein: said combustion gases are supplied in a ratio offrom about 0.05 to 5.0 parts recycled combustion products to 1 part newcombustion products.
 8. The subject matter of claim 5 wherein said newcombustion products are generated by first producing an ash free gaseousfuel through reaction of a coal derived fuel with all or a portion ofsaid combustion gases that are recycled, and burning said ash freegaseous fuel with oxidant in a combustion chamber ahead of the MHDgenerator.
 9. The subject matter of claim 5 wherein said new combustionproducts are formed by reaction of a coal derived fuel with preheatedoxidant in a combustion chamber designed to separate out a majorfraction of the ash.
 10. The subject matter of claim 5 wherein therecycled combustion products are introduced into the combustion chamberin which reaction of fuel and oxidant is taking place, so that mixing ofsaid recycled gases and chemical reaction of fuel and oxidant take placesimultaneously and in the same chamber.